Even in the technology-intensive world of medicine, there’s a lot to be said for simplicity. A new study [PDF] from the World Health Organization (WHO), which monitored 340 locations in five countries for five years, finds that bed nets treated with pesticide continue to stop malaria transmission, even as mosquitoes develop pesticide resistance. The report was presented at the annual meeting of the American Society of Tropical Medicine & Hygiene (ASTMH).
Mosquito nets have literally been around for ages; the Greek historian Herodotus noted their use in Egypt as early as the 5th century BCE. For all that time, they were pretty effective—certainly effective enough that people kept using them—but that efficacy got a boost in the mid-20th century when we started spraying them with pesticide. The year 2000 saw the introduction of the long-lasting insecticidal net (LLIN), an inexpensive bed net made with insecticide-treated fabric that drove down the number of malaria cases even further.
But heavy insecticide use has its costs. Pesticides are similar to antibiotics, in that they can’t kill every single one of the species they’re meant to destroy. The survivors reproduce, creating new generations that are able to resist the treatment. And the more we use, the faster they can adapt. We’re now facing a crisis of antibiotic resistance, and pesticide resistance is not far behind. Mosquitoes in 60 countries have already developed a resistance to the pesticides used in LLINs.
Sven Torfin / WHO 2016
Consequently, researchers at the WHO’s Global Malaria Programme were concerned that the rise of pesticide-resistant mosquitoes would create a decrease in LLIN effectiveness. They spent five years surveying LLIN use and pesticide resistance in 340 sites in malaria-heavy Benin, Cameroon, India, Kenya, and Sudan.
The results were surprisingly positive. People who used LLINs around their beds at night were significantly less likely than others to become infected. From 2000 to 2015, the WHO estimates, interventions like LLINs prevented around 663 million new malaria infections in sub-Saharan Africa. And of those potential cases, 69 percent were prevented by LLINs.
Co-author Tessa Knox of the WHO notes that the potency of LLINs comes not from the pesticide or the net alone, but from their combined power. “A resistant mosquito may not die immediately after landing on a net, but it could continue to absorb insecticide as it seeks a way to get through and bite a person beneath the net,” she said in a statement. “This may eventually kill the mosquito and stop onward transmission of malaria parasites.”
Encouraging though these findings may be, experts caution that there’s still a lot more work to do as pesticide resistance continues to spread.
Stephen Higgs is president of the ASTMH. “This study provides encouraging news that we have not yet run out of time in battling insecticide resistance,” he said in the statement. “However, we must take advantage of the time we now have to invest in research and generate new tools that will allow us to finally defeat this complex and challenging disease.”
A handful of those new tools are already in the works. Some researchers are exploring chicken feathers as a natural mosquito repellent, while others are developing high-tech pills that could deliver a week’s worth of malaria medication with one swallow.